In the production of monolithic circuits, there is the need, as progress occurs in the direction of applications at higher and higher frequencies, to decouple passive circuit elements from the substrate. One way to guarantee such decoupling is to provide a sufficiently thick insulating layer between the circuit element and the substrate. Such an insulating layer is known from Erzgräber, Grabolls, Richter, Schley, and Wolf, “A Novel Buried Oxid Isolation for Monolithic RF Inductors on Silicon,” IEEE 1998.
For the large area of semiconductor technology based on silicon, this means to implement the passive circuit elements such as conductive connections, resistors, capacitors, and inductors on an insulating layer that is on the same level as the silicon substrate, in other words with the active circuit elements. The great success of silicon semiconductor technology, in particular, is based on the possibility of converting silicon, which is a semiconductor, to a high-quality insulator, namely SiO2, by means of a simple chemical oxidation step. In this way, active semiconductor elements that work insulated from one another can be integrated into electronic systems, at a high packing density, in an inexpensive manner. The chemical process of oxidation usually takes place at atmospheric pressure and at temperatures in the range of 800 to 1150° C. In this regard, the chemical conversion from Si to SiO2 proceeds from the surface and progresses into the depth of the silicon substrate. The supply of oxygen to the front of the chemical reaction takes place via diffusion. Oxide that builds up hinders the diffusion process as it increases in depth; the oxidation speed decreases and becomes increasingly inefficient. Oxide layers such as those that are commonly produced in CMOS technology lie in the range of less than 1 μm. The circumstance that an increase in volume goes along with the conversion of Si to SiO2 is of technological significance. Finally, the thickness of the SiO2 layer that has formed measures approximately 2.2 times that of the silicon layer from which it was formed.
Geometric requirements with regard to monolithic integrated SiO2 insulating layers are, on the one hand, their expanse into the depth, whereby an effective decoupling from the substrate is achieved at several μm to several 10 μm. On the other hand, lateral expanse values of up to several mm2 are necessary, in order to be able to place passive components. For greater depth, trench structures are first produced in the silicon substrate, by means of an anisotropic etching process. Remaining silicon ridges can then be converted to oxide in the oxidation process, from all sides that are accessible to oxygen. If the ratio of the width of the ridge to the width of the etched gap is selected in such a manner that after oxidation, the interstices have been closed as a result of the volume increase, the desired effect of the thick insulating layer will have been achieved. However, real production processes cannot be controlled with a specific desired degree of accuracy. A certain distribution of the ridge width occurs over the work piece (silicon wafer) and over several work pieces, with the effect that either part of the gap remains open, or the ridge is not completely converted to oxide. For this reason, the ratio of ridge to gap is dimensioned, right from the start, in such a manner that after oxidation, a gap remains, which is then filled up, in a further process step, with oxide from a chemical vapor deposition. Differences in the thermal expansion coefficients between monocrystalline silicon and SiO2 as well as the required process temperatures lead to mechanical stresses and therefore to warping of the geometry of the ridges. Thus, for example, ridges that were originally arranged to be parallel over long distances demonstrate cluster formation after oxidation, with ridges that are inclined towards one another. These clusters distinguish themselves from the adjacent cluster by a relatively wide gap. The latter gaps are too wide to be filled up by means of oxide deposition, and this is disadvantageous for the quality of the insulating layer that is formed.